U.S. patent number 4,724,823 [Application Number 06/912,571] was granted by the patent office on 1988-02-16 for radiant gas burner assembly.
This patent grant is currently assigned to Solaronics, Inc.. Invention is credited to Thomas W. Simpson.
United States Patent |
4,724,823 |
Simpson |
February 16, 1988 |
Radiant gas burner assembly
Abstract
A gas fired radiant griddle assembly employing a series of
perforated ceramic tiles to define a burner surface positioned
beneath the griddle. The center area of the assembled ceramic tiles
is blocked off so as to define an annular burner surface to
minimize the temperature gradient across the griddle surface and
the hot combustion effluents are drawn by convection beneath the
griddle surface and out the rear corners of the combustion chamber
beneath the griddle to heat the peripheral and corner portions of
the griddle and further minimize the temperature gradient across
the expanse of the griddle.
Inventors: |
Simpson; Thomas W. (Rochester,
MI) |
Assignee: |
Solaronics, Inc. (Rochester,
MI)
|
Family
ID: |
25432134 |
Appl.
No.: |
06/912,571 |
Filed: |
September 29, 1986 |
Current U.S.
Class: |
126/39R; 99/422;
99/447; 126/39D; 126/39E; 126/39H; 431/1 |
Current CPC
Class: |
F24C
3/047 (20130101); A47J 37/0682 (20130101); F24C
3/126 (20130101); F24C 3/103 (20130101); F24C
3/067 (20130101) |
Current International
Class: |
A47J
37/06 (20060101); F24C 3/12 (20060101); F24C
3/00 (20060101); F24C 3/06 (20060101); F24C
3/10 (20060101); F24C 003/00 () |
Field of
Search: |
;126/52,39R,39BA,39D,39E,39H,39N,39J,39K,214A,214R ;99/422,447
;431/1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Jones; Larry
Attorney, Agent or Firm: Krass & Young
Claims
I claim:
1. A radiant gas burner assembly comprising:
a planar metal member defining a heat utilization surface;
generator means defining a radiant heat burner surface spaces
substantially centrally of and below said planar member for
directing radiant energy to said planar member as a primary heating
source therefor;
a housing defining a closed combustion chamber around and above
said burner surface and peripherally around and below said heat
utilization surface; and
means for directing a current of combustion effluent to a
peripheral location of said chamber to act as a secondary heating
source for at least one peripheral portion of said planar
member.
2. A radiant gas burner assembly according to claim 1 wherein:
said burner surface is quadrangular.
3. A radiant gas burner assembly according to claim 2 wherein:
said housing is open at the top and generally rectangular;
said planar member comprises a griddle positioned over and upwardly
closing said combustion chamber; and
said directing means includes an exhaust conduit extending along at
least one side wall of said housing and opening in said combustion
chamber adjacent at least one corner of the chamber.
4. A radiant gas burner assembly according to claim 3 wherein:
said radiant heat burner surface is defined at the upper surface of
a ceramic tile structure including vertical perforations;
said generator means further includes a plenum chamber beneath said
ceramic tile structure and means for introducing a gas/air mixture
into said plenum chamber for upward passage through said
perforations and combustion at said burner surface at the upper
surface of said ceramic tile structure; and
the combustion effluent generated at said burner surface are drawn
by convection laterally out of said combustion chamber and into
said conduit at said openings
5. A radiant gas burner assembly according to claim 4 wherein:
said gas/air mixture introducing means includes a mixing tube
passing through the front wall of said plenum chamber; and
said exhaust conduit extends along the rear side wall of said
housing and communicates with the two rear corners of said
combustion chamber.
6. A radiant gas burner assembly according to claim 5 wherein:
said housing includes a bottom wall;
said ceramic tile structure is sealingly positioned in said bottom
wall centrally beneath said griddle; and
said annular burner surface is produced by blocking the
perforations in the central region of said ceramic tile structure
while leaving the perforations in the annular periphery thereof
exposed.
7. A radiant gas burner assembly according to claim 6 wherein:
said ceramic tile structure includes a plurality of individual
tiles positioned in side by side relation in said bottom wall of
said housing.
8. A radiant gas burner assembly according to claim 5 wherein:
said housing, griddle, and plenum chamber define a closed sealed
volume having only a single entrance defined by said mixing tube
and at least a single exit defined by an opening in said housing
wall at the periphery of said combustion chamber.
9. A radiant gas burner assembly comprising:
housing means defining a sealed column including an upper metal
planar member defining a heat utilization surface and forming the
upper boundary of said volume, a planar perforated ceramic tile
structure to serve as a radiant heat source and positioned
substantially centrally of and below said heat utilization surface
to direct radiant energy toward said heat utilization surface, a
plenum chamber positioned below said ceramic tile structure and
defining the lower boundary of said volume, and side defining the
lower boundary of said volume, and side walls of said volume above
and around said ceramic tile structure;
a mixing tube passing through the front wall of said plenum chamber
and constituting the only entrance to said sealed volume; and
at least one opening in a wall of said housing adjacent the
periphery of said housing whereby combustion air entering said
volume through said mixing tube in a gas/air mixture flows upwardly
through the perforations in said ceramic tile structure for
combustion at the upper face of said ceramic tile structure and
thereafter directly toward said peripheral opening to serve as a
secondary heat source for the heat utilization surface in the
immediate area of said opening.
10. A radiant gas burner according to claim 9 and further
including:
an exhaust conduit extending along any side wall of said casing and
communicating with said opening in said wall to provide an exhaust
passage for said combustion effluent.
11. A radiant energy griddle apparatus comprising:
a substantially planar metallic griddle member having a cooling
area;
means defining a chamber having a bottom wall, side walls and an
open top; said griddle member being sealingly disposed on the
chamber means to close said top;
a radiant generator having a lower surface and a perforate active
upper surface area disposed in spaced relation with the griddle
member; said generator active surface area being substantially
smaller than and wholly laterally inwardly spaced from the
perimeter of said cooking area;
plenum means adjacent the lower surface of said generator for
receiving fuel and air for passage through the generator and into
said chamber;
and exhaust means having at least one port at the periphery of the
chamber and adjacent a periphery of said cooking area for drawing
effluent from the radiant generator, along the lower surface of
said griddle member to said port to supplementally heat the griddle
member in the vicinity of said port.
12. An apparatus as defined in claim 11 wherein said active area
defines at least a substantially closed figure having an inactive,
nonradiating center.
13. Apparatus as defined in claim 11 wherein said active area is of
non-uniform.
14. Apparatus as defined in claim 11 further including igniter and
flame sensor means disposed above and adjacent said active surface,
the combination further including a conductive mesh ground element
disposed surroundingly and in spaced relationship to said igniter
and flame sensor means and grounded to the housing means defining
said chamber.
15. Apparatus as defined in claim 11 further including multiple
exhaust ports at the peripheral corners of said chamber.
16. A radiant fired griddle comprising:
a griddle member having a lower surface and an upper cooking
surface;
a support housing having peripheral walls terminating in an upper
edge, said griddle member being sealingly disposed on said upper
edge;
a radiant generator forming a bottom wall of said chamber of said
housing and being connected to said peripheral wall in sealed
relationship, said generator comprising, in part, a perforate tile
pattern and, in part, a non-radiating center portion;
means for admitting fuel and air to the lower surface of the
perforate tile pattern whereby said fuel and air passes into the
chamber formed by the radiator, the peripheral wall and the griddle
member;
means for igniting said fuel and air mixture adjacent the surface
of the tile pattern within said chamber thereby to activate the
generator; and
exhaust means formed in said wall at at least one peripheral
location to force the flow of effluent from the active surface of
the tile to a peripheral location to boost the temperature of the
griddle member above said peripheral location thereby to promote
temperature uniformity of said griddle member over the entire
surface area thereof.
17. Apparatus as defined in claim 16 including valve means for
supplying gas to the generator, switch means for turning said valve
on and off, and modulator means for modulating the proportion of a
basic time cycle during which said gas valve is on.
18. A radiant fired griddle apparatus comprising:
a rigid metallic griddle having an upper cooking surface and a
lower energy-receiving surface;
a gas-fired, radiant energy generator disposed below and in spaced
relationship to said energy-receiving surface;
means for exhausting products of combustion from said
generator;
valve means for controlling the flow of combustible gas to the
generator; and
control means for operating said valve in a mode wherein said valve
is opened for a predetermined portion of a repeating time cycle.
Description
FIELD OF THE INVENTION
This invention relates to gas-fired radiant generators and more
particularly to a griddle-topped food cooking unit having one or
more gas fired radiant heat generators.
BACKGROUND OF THE INVENTION
Radiant gas generators for cooking applications have been proposed
in various combinations including deep fryers (U.S. Pat. No.
4,397,298) and ovens (U.S. Pat. No. 4,480,628). The burners or
generators in these combinations are of the type in which the
burner surface is defined by a ceramic tile structure including a
plurality of parallel perforations in the tile structure and
wherein a fuel gas mixture is burned at the upper surface of the
tile structure to generate the radiant heat used in the cooking
process. Whereas the radiant heat burners of the ceramic tile type
have been generally satisfactory for general cooking applications,
they suffer from the disadvantage that the generated heat tends to
peak at the geometric center of the tile surface so that the
overlying heating surface is relatively hot adjacent the center of
the surface and relatively cool adjacent the perimeter of the
surface with the result that the cooking performance of the heating
surface varies considerably across the expanse of the heating
surface. While this may not represent a problem in deep fryers and
ovens, it may present considerable use problems in cooking on a
griddle, especially in large batch quantities.
SUMMARY OF THE INVENTION
This invention is directed to the provision of a radiant gas burner
assembly in which the temperature of a cooking surface heated by
the burner is substantially uniform over the surface area
thereof.
The radiant gas burner assembly of the invention comprises a
griddle member defining a heat utilization surface; generator means
defining a source of radiant heat spaced below the griddle member;
a housing defining a closed chamber above the generator and below
the heat utilization surface; and means for drawing heated effluent
by convection to peripheral areas of the chamber beneath the
griddle thereby to supplement or boost griddle temperature at the
edges and corners for temperature uniformity. This arrangement has
the effect of drawing the hot combustion effluent beneath the
peripheral portions of the heat utilization surface to minimize the
temperature gradient across the heat utilization surface.
According to a further feature of the invention, the radiant heat
burner surface is constructed so as to have a non-radiating center
area so as to minimize the concentration of radiant energy in the
central region of the griddle positioned to receive the energy and
thereby reduce the heat gradient across the expanse of the heat
utilization surface.
According to a further feature of the invention, the annular burner
surface is defined by a ceramic tile structure including parallel
perforations wherein the perforations adjacent the central region
of the tile structure are blocked or non-existent, leaving the
peripheral perforations open to define the actual radiating
area.
According to a further feature of the invention, a plenum chamber
is positioned below the ceramic tile structure; the casing is
rectangular and coacts with the plenum chamber and upper planar
member to define a sealed volume; a mixing tube passes through the
front wall of the plenum chamber and constitutes the only entrance
to the sealed volume; and openings are provided in a wall or walls
of the housing, preferably adjacent the front and/or rear corners
of the housing and constitute the only exit or exits from the
sealed volume through an opening or openings which may be
positioned to draw the hot effluent of combustion to griddle areas
requiring heat boost for temperature uniformity. With this
arrangement, an air/fuel mixture enters the sealed volume through
the mixing tube and flows upwardly through the perforations in the
ceramic tile structure for combustion at the upper face of the
ceramic tile structure, whereafter the combustion effluent leaves
the sealed volume through the exhaust openings of the casing. Hot
combustion effluent is thus moved continuously and efficiently in
the areas needed, under the heat utilization surface, to minimize
the temperature gradient across that surface.
In one disclosed embodiment of the invention, the upper planar
member comprises a griddle sealing the open upper end of the
housing; the ceramic tile structure includes a plurality of
individual perforated tiles positioned in side by side relation
with the perforations in the central portion of the tile area
blocked to define an annular burner surface at the perimeter of the
tiles; and an exhaust conduit extends along the rear side wall of
the casing and communicates with the two rear corners of the
combustion chamber to facilitate the continuous removal of effluent
from the combustion chamber through appropriate areas suitably
positioned. In another embodiment, tiles are arranged in a closed
figure around a central steel plate so that, again, the central
portion of the overall burner area is non-radiating.
BRIEF DESCRIPTIONS OF THE DRAWINGS
FIG. 1 is a perspective view of a gas fired radiant heat griddle
according to the invention with the griddle plate exploded
upwardly;
FIG. 2 is a fragmentary cross-sectional plan view of the griddle of
FIG. 1;
FIGS. 3 and 4 are cross-sectional views taken respectively on lines
3--3 and 4--4 of FIG. 2;
FIG. 5 is a schematic view of the control circuit for the
griddle;
FIG. 6 is a perspective view of an igniter assembly with a ground
screen;
FIG. 7 is a plan view of an alternative generator surface
arrangement; and
FIG. 8 is a circuit diagram for a device having a low temperature
"hold" selector.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The embodiment of the invention shown in the drawing includes a
griddle plate 10; a housing 12; independently controllable left and
right generator assemblies 14 and 16; an exhaust conduit assembly
18; and a control assembly 20.
Griddle plate 10 includes a left portion 10a and a right portion
10b divided by a central rib 10c and including an outer peripheral
lip or flange 10d extending totally around the griddle and defining
a downwardly opening peripheral seat or groove 10e. Griddle 10 is
preferably formed as a heavy cast aluminum member.
Housing 12 is preferably formed of 20 gauge aluminized steel and
includes a front side wall 12a, a left side wall 12b, a back side
wall 12c, a right side wall 12d, a bottom wall 12e, and a central
partition 12f dividing the housing into a left compartment 12g and
a right compartment 12h. Groove 10e of flange portion 10d of
griddle plate 10 fits sealingly over the upper edge 12i of the side
wall of the casing and partition portion 10c sealingly engages
housing partition 12f so that the griddle plate sealingly encloses
the open upper end of the housing. To insure a good seal which is
essential to the operation, a gasket 15 of woven ceramic tape is
applied to the upper surfaces of 12a, 12b, 12c, 12d and to the
central partition 12f. Gasket 15 or a seal of equal effectiveness
is necessary to insure a positive draft in the burner venturi 26
hereinafter described and to avoid back pressure above the
burner.
Generator assemblies 14 and 16 are identical and are respectively
disposed in the bottom wall of compartments 12g and 12h. Only
generator assembly 16 will be described and it will be understood
that assembly 14 may be identical to or the mirror image of
assembly 16.
Generator 16 includes a plurality of contiguous and coplanar
ceramic tiles 22, a plenum chamber 24, and a venturi mixing tube
26. Tiles 22 are arranged in side by side abutting relation and
each include a plurality of perforations 27 (FIG. 4) extending from
the bottom surface to the top surface of the tile. There may, for
example, be 200 perforations per square inch of tile surface with
each perforation having a diameter of 0.05 inches so that
approximately 1/3 of the surface area of the tile constitutes open
area. The central area of the assembled tiles is blocked by a layer
of ceramic cement 28 applied in paste form to the undersurface of
the assembled tiles so as to preclude upward movement of fuel air
mixture through the perforations of the central area of the
assembled tiles and allow the fuel air mixture to move upwardly
through the perforations in the annular perimeter 22a of the
assembled tiles; i.e., the only active burner area of the tiles is
a rectangular perimeter strip 22a about 11/4" in width around the
outside of the composite of tiles. It may be advantageous to vary
the rectangular surface in certain instances or designs to obtain
the required temperature uniformity. As a first alternative, the
portions of the tiles which make up the blocked area may be left
unperforated. As a second alternative, rectangular tiles may be
placed around a central steel plate.
Plenum chamber 24 is defined as a dish like sheet metal structure
underlying the assembled tiles and including a bottom wall 24a,
side walls 24b, and an upper lip portion 24c coacting with the
bottom 12e of the casing to define a groove 24d receiving
peripheral flange portions 22b selectively provided at the external
peripheries of the assembled tiles.
Mixing tube 26 extends through the front side wall of plenum
chamber 24 and includes a main body portion 26a receiving a gas
supply line 28 and a venturi portion 26b positioned within the
plenum chamber and beneath the assembled tiles. Clearly, other
forms of introducing the gas/air mixture into the burner can be
used. Such, for example, as pressurized air mixing.
Exhaust conduit assembly 18 is formed of a suitable sheet metal
material and includes a conduit 18a extending across the rear wall
12c of the housing and a central exhaust vent or stack 18b
communicating centrally with conduit 18a. Conduit 18a communicates
with the rear corners of griddle chamber 12h through openings 12j
and 12k and with the rear corners of griddle chamber 12g through
openings 12l and 12m. Conduit 18a further includes corner portions
18c and 18d respectively wrapping around the left rear corner of
griddle chamber 12g and the right rear corner of griddle chamber
12h. Conduit portion 18c communicates with the left rear corner of
chamber 12g through an opening 12n in casing side wall 12b, and
conduit portion 18d communicates with the right rear corner of
griddle chamber 12h through an opening 12p in casing side wall 12d.
The ducts are not necessarly limited in position to that shown.
They may be disposed as desired to obtain the uniform temperature
distribution desired and their location ties in with the shape and
size of the radiant area to achieve the maximum uniformity.
Control assembly 20 includes a control panel 30; a main solenoid
valve 32; a left solenoid valve 34; a right solenoid valve 36; a
left igniter assembly 38; a right igniter assembly 40; a left
burner toggle switch 42; a right burner toggle switch 44; a main
switch 46; and a junction box 48 receiving outside power through a
suitable cable 50.
Each igniter assembly 38, 40 includes an igniter electrode 52
positioned over the exposed annular burner surface 22a of the
assembled tiles, a ground electrode 54, and a flame probe or sensor
electrode 56 also positioned over the annular burner surface
22a.
If the burning area of the tile is severely limited with respect to
the total area, the ground area can conveniently be increased by
putting an Inconel mesh screen 57 over the electrode wire ends and
grounding it to the housing with sheet metal screws as shown in
FIG. 6. Exclusive of the screen 57, the igniter assembly 52, 54, 56
is a known device in which an alternating voltage of about 60 volts
is applied between the electrode 56 and ground. The presence of a
flame ionizes the air surrounding the electrode and rectifies the
current flow to ground. This rectified dc current is sensed and
employed to hold a relay open which continues gas flow. The screen
57 adds to the ground plane and improves the operation of the flame
sensor during low temperature operation just after ignition.
In use, the griddle assembly is positioned within a suitable
countertop (not shown) with the upper surface of the griddle plate
10 positioned generally flush with the countertop and with the
control panel 30 positioned beneath the forward edge of the
counter.
In operation, and assuming that it is desired to use the right hand
griddle unit, main switch 46 is actuated and right hand burner
control switch 44 is actuated to open solenoid valve 36 and supply
gaseous fuel under pressure from a fuel source 58 through line 28
and nozzle 29 to mixing tube 26 where it unites with air flowing
under natural aspiration through the annular entry space defined
between the mixing tube and the nozzle and flows through venturi
portion 26b into the plenum chamber beneath the tile assembly 22.
The gas and air mixture then flows upwardly through the exposed
perforations 27 in the annular burner area 22a to the top surface
of the tiles where it is ignited by igniter electrode 52 coacting
with ground wire 54. Ultimately the flame heats the tile cherry red
to produce a radiant flame surface throughout the annular burner
surface area 22a; i.e., the center is unheated and does not
radiate.
The heat generated by the exposed burner surface 22a radiates
upwardly to heat griddle portion 10b. The annular burner surface
22a underlying the griddle surface 10b has the effect of minimizing
the temperature gradient across the griddle surface as compared to
the gradient that would be established with a total area burner
tile surface since the radiation intensity is a trigonometric
function with the highest intensity occurring directly
geometrically over the flame front and the intensity lessening in a
direction away from the location immediately over the flame front
by an amount corresponding to the cosine of the angle between the
ray emanating directly upwardly from the flame and the ray
emanating to the particular point in question on the griddle
surface. Thus, rather than having a very high temperature in the
center of the grill and a significant temperature gradient in all
directions moving away from the center of the grill, the described
annular burner surface has the effect of establishing a relatively
uniform temperature across the expanse of the griddle surface.
The combustion effluents moving upwardly from the tile surface move
rearwardly by convection beneath the griddle surface and exit from
the chamber 12h through corner openings 12j, 12k and 12p for entry
into conduit 18a and discharge through stack 18b. The hot effluents
moving beneath the griddle surface and outwardly through the
corners of the chamber 12h have the effect of boosting the heat at
the perimeter and corner portions of the griddle to minimize the
temperature gradient across the expanse of the griddle.
It will be seen that the housing 20, griddle 10, and plenum chamber
24 coact to define a sealed volume having only a single inlet
constituted by the mixing tube 26 and a single outlet through the
rear corners of the combustion chamber. This arrangement
facilitates control of the combustion process and also has the
effect of creating a strong, directed convection current flow
beneath the griddle and out of the direct openings of the
combustion chamber.
Griddles constructed in accordance with the present invention and
incorporating the annular burner surface in combination with the
described convection current flow provide a temperature gradient
across the griddle surface of no more than 3 degrees F. as opposed
to prior art designs in which temperature gradients of 50 degrees
F. or more are commonly encountered across the expanse of the
griddle surface.
Looking now to FIG. 7 an alternative embodiment of the invention
particularly useful in connection with vary large griddle surfaces
is disclosed. In the FIG. 7 embodiment one half of a double griddle
arrangement is shown to comprise a housing 60 having an interior
horizontal bulkhead 62 into which is sealingly installed a
rectangular pattern 64 of individual rectangular or square burner
tiles. The center area 66 is made of sheet steel just as the
bulkhead floor area 64 is constructed of sheet steel. In this
embodiment a good seal must be maintained around the entire
peripheries, both inside and outside, of the pattern 64. Area 68 of
the pattern 64 is perforated to permit the flow of the fuel air
mixture through from the plenum which, as will be understood from
the previous description lies below the plane of surfaces 62 and
66, through the burner tiles. Areas 70 are blocked off by paste as
previously described. Area 71 is left unblocked to provide a
location for the igniter and flame sensor electrodes 52, 54,
56.
The housing 60 further comprises walls 72, 74, 76 and 78 to define
exhaust ports 80, 82, 84 and 86 of which ports 80 and 82 are
located at the rear corners and ports 84 and 86 are located at the
front corners. Again the location of the exhaust ports, although
shown diagrammatically, is apparent from the planned view and
differs from the foregoing embodiment primarily in the fact that
ports 84 and 86 are located in the front corners of casing 60.
Referring now to FIG. 8 a second embodiment of the control
circuitry is disclosed, the principal differences between the
circuit of FIG. 8 and the circuit of FIG. 5 lying in the provision
of a "hold" mode of burner operation wherein a low or intermediate
temperature is maintained between full-on cooking times.
In this connection it will be noted that neither of the embodiments
of FIGS. 5 nor 8 provides temperature control through gas valve
modulation. Of course, the invention may be incorporated into a
unit having such temperature control but it is believed preferable
in certain applications to provide simple preset gas flow controls
such that operator responsibility is reduced to the decision as to
whether to turn the unit on or off or, in the case of the
embodiment of FIG. 8, to place the unit in the low or intermediate
temperature "hold" mode.
Looking specifically to the circuit diagram of FIG. 8 it will be
noted that the switch 90 is a double throw double pole switch with
center "OFF" position and is provided with switch elements 92 and
94. It will be appreciated that switch 90 corresponds in physical
location with either of switch 42 or switch 44 of FIG. 1. Element
96 can be moved from the "off" position shown to a "cook" position
where it contacts terminal 96 and to a "hold" position where it
contacts terminal 100. Switch element 94 can be moved from the
"off" position shown to a "cook" position corresponding to terminal
98 and to a "hold" position corresponding to terminal 102. Switch
elements 92 and 94 are tied together so that they must move in
synchronism.
The embodiment of FIG. 8 further includes the circuit board 104
which is essentially a sequencer to provide internal connections to
the various elements of the circuit, and a timer 106. Additional
elements which differ from the circuit of FIG. 5 include the "hold"
light 108 which comes on when the switch 90 is placed in the "hold"
position. A "purge" light 110 is provided to indicate that the
sequencer of the circuit board 104, a known device, is in the 10
second purge mode; i.e., after the switch 90 is placed in either of
the cook or hold positions, a relight sequence including a 10
second purge of the combustion chamber and a subsequent activation
of the igniter electrodes 52, 54 and 56 is carried out in a manner
well known and well understood by those skilled in the art. A light
112 is provided to show that the gas valve is actually open and the
unit is heating.
When the switch 90 is in the "cook" position power is supplied
directly to the circuit board 104 through terminal and 98 and line
114. Accordingly the circuit board operates to perform the purge
cycle and thereafter supplies power to the igniter and maintains
the gas valve in the open position as previously described. When
the switch 90 is placed in the "hold" position, power is applied
through terminal 100 to the timer 106 through line 116 and, from
the timer 106 to the switch at terminal 102 to terminal 94 and
thence, as before by line 114 to circuit board 104. Accordingly, in
the "hold" position the circuit board 104 receives power only
through the timer 106 and it is the function of the timer to
establish a base cycle and to allow the gas valve and the burner to
function only for a portion of the base cycle. For example, a base
cycle 130 seconds may be established and a portion of 20 seconds
may be set aside for activation of the burner, leaving 10 seconds
for the purge as previously described. As a result the burner is on
for 20 seconds out of every 130 seconds and this is a form of
temperature control using time duration modulation.
Whereas preferred embodiments of the invention have been
illustrated and described in detail, it will be apparent that
various changes may be made in the disclosed embodiments without
departing from the scope or spirit of the invention.
* * * * *